Processing of calcined pigments



Pa cea, 1938 I r I 2,131,841

PROCESSING OF CALCINEIl PIGMIENTS Sanford C. Lyons, Bennington, Vt., asslgnor to Bird Machine 00., Walpole, Mass, a corporation of Massachusetts No Drawing. Application May 10, 1937, Serial No. 141,710

15 Claims. (CL'134-58) This invention relates to the processing of Oxide in p po s y, about eachcalcined pigments with a view toward arriving The ore is digested with strong sulphuric acid at a finished pigment product of such fineness serving to transform the mixed oxides into iron of particle size, freedom from water-soluble conand titanium sulphates. The cake of mixed 5 taminations, and other qualities as to render such iron and titanium sulpha thus Produced is 5 product eminently'usefulfor various purposes, dissolved in water and the mixed salt solution for instance, in paints, lacquers, resins, and other is then treated with scrap iron or its equivalent compositions. to reduce the ferric sulphate to ferrous sulphate In producing pigments for use in paint and while the titanic sulphate is preserved largely as lacquer positions, it is customary tocalcine such. After silica and other water-insoluble im- 10 the pigment particles, which are of extremely purities have been settled from the resulting fine or colloidal particle size by reason of their solution of ferrous sulphate and titanic sulphate, previous formation through precipitation, so as the solution is filtered to produce a clear liquor to develop particles or aggregates of sufliciently which is then heated to boiling for the purpose is larger dimensions to aiford maximum opacity of causing the decomposition by hydrolysis of or covering power. In this latter connection, it the titanic sulphate into titanic hydroxide. The might be noted that, when a pigment, e; g., titanic hydroxide is precipitated as extremely titanium dioxide, is formed through preeipitafine or colloidal particles, a substantial fraction tion, a substantial fraction of the pigment parof which are smaller than even the wave length ticles are apt to be of unduly fine particle size, of light just beyond the visible range, as already 20 namely, of a size less than the wave length of indicated. The precipitated titanic hydroxide is light just beyond the visible range, e. g., a wave washed free of iron salts during filtration or length, 0.15, within the ultra-violet region, in afterwards; and the washing treatment may be consequence of which the pigment" does not exso thoroughly performed {as to leave as little as,

" hibit its maximum potential light-reflecting or say, about 0.01% iron, calculated as oxide, in 25 covering property. 'It is for this latter reason the washed product. Such residual .riron salt as that pigment production includes a calcining remains in the washed product is sometimes reoperation, in the course of which the ultra-fine duced to soluble ferrous salt by treatment of or colloidal pigment particles are caused to clussuch product with suitable reducing agents, such ter into aggregates distinctly larger than such as hyposulphurous acid or hyposulphite contain- 30 wave length of light just beyond the visible range. ing, if desired, powdered zinc dust or its equiva- Inasmuch as the growth of pigment particles lent, to accelerate the solubilization and decoloriduring the calcining operation cannot be conzation of residual iron salt. After the decoloriztrolled with precision or accuracy, it is invariably ing treatment, the precipitated titanichydroxide the case that the pigment emerging from the calmay be washed and filtered from suspension in 35 cining. kiln contains a substantial fraction of the form of a thick paste or plastic mass ready undesirably large pigment clusters or aggregates, for the conventional calcining operation. that is, clusters or aggregates so coarse and gritty Preparatory to calcining the precipitated tias to detract from the smoothness, homogeneity tanic hydroxide, it is desirable toadmix there- 40 of texture, and covering power potentially realwith a suitable fiuxing agent, such as'potassium 40 izable from the pigment. carbonate, which promotes growth or aggregation Theinvention hereof will be discussed in terms of the ultra-fine or coloidal particles to sizes disof the processing of titanium dioxide as it emerges tinctly greater than the wave length of light from a calcining kiln, but it is to be understood iustbeyond the visible range, as previously indithat the inventive principles hereof are applicated. This may be done by admixing with the 45 cable to other pigments prepared by processes paste or plastic mass of titanic hydroxide sufiiinclusive oi." precipitating and calcining operacient potassium carbonate solution of ap ropritions. Before entering into a discussion of the ate strength to yield a thick slurry and removing invention hereof, it is perhaps helpful to'outline free solution from the slurry to form a paste the'prod'uction of titanium dioxide pigment up for introductioninto the calcining kiln. Dur- 'to and including the precipit g and calcining ing calcination, the titanic hydroxide is substanoperations. tially completelydehydrated as the fine, ultra- The common source of titanium dioxide pigfine, and colloidal particles formclusters or agment is the ore known as ilmenite, whichis comg'regates of titanium dioxide of promiscuous sizes '55 posed essentially of iron oxides titanium distinctly greater-than the wave length of light within the ultra-violet region wherefore, the calcined product emerging from the kiln consists of titanium dioxide. pigment, a substantial fraction of whose particles are undesirably gritty or oversized for lacquers, paints, and other compositions. It is hence generally the custom to reduce the calcined pigment as'an aqueous slurry to finer particle size in a ball mill and to deliver the ball-milled slurry to grading or. classifying apparatus serviceable for resolving the slurry into a fine pigment fraction for use in lacquers, paints,

or the like and a relatively coarse or gritty pig- .ment fraction for retreatment in the ball mill.

' However, the grading or classifying apparatus used in prior art practice has not only been cumbersome but-has been imperfect, so far as concerns yielding a fine pigment fraction with the desired selectivity or freedom from oversized p'articles. In other words, it has yielded fine pigment fractions wherein there was still likely to be present considerable oversized or coarse pigment particles detracting from the smoothness, uniformity of texture, opacity or tinctorial power,- and other qualities desired in lacquers or paints whereinto such fine fractions entered.

In accordance with the present invention; the calcined titanium oxide emerging from the calcining kiln is suspended in water to form a thick suspension, preferably in the-substantial absence of any dispersing agents. The resulting suspension, which may well have a solids content of, say. about 30% to is progressively fed into a ball mill which reduces to finer particle size a considerable portion of the oversized or gritty pigment particles. The ball-milled suspension progressively leaving the ball mill may be passed as such or after dilution with water to a centrifuge, preferably a continuous flow centrifuge of the type shown in Lyons application Serial No. 89,886, filed July 10, i936, which centrifuge is designed to receive at substantially constant rate or volume of flow an incoming stream of a solids suspension to be centrifuged as solids deposited from suspension on the centrifuge wall are being progressively emitted through one end of the centrifuge and liquid .containing residual finer solids in suspension or practically devoid of solids is being emitted through the other end of the centrifuge. As further stressed in this application, centrifugation is performed under substantially constant solids-depositing conditions, in-

cluding substantially constant effective centrifuging-force, since the solids deposited on the centrifuge wall are progressively removed during the centrifuging operation and the rate-of flow of suspension through the centrifuge and other significant factors are maintained substantially obnstant during the centrifuging operation. Accordant withthe instant invention, the centrifuge is operated under combined conditions conducive to the progressive emission from the centrifuge of the oversized or gritty pigment particles, preferably particles of a size greater than about 2 microns, while the liquid effluent from the centrifuge consists of an aqueous suspension containing the pigment particles of a size finer than I about 2 microns but, nevertheless, distinctly the centrifuge substantial freedom from relatively coarse and/or gritty pigment particles, that is.

particles of a size of, say, 5 or 6 microns orgreater,

whereas. on the other hand, the relatively coarse Thus, such suspension may be treated with a solids fraction emitted from the centrifuge contains entrained therein but an insignificant amount of pigment particles of a size smaller than about Z'microns. It is thus seen that the centrifugal fractionating treatment performed hereby on the ball-milled pigment slurry is a highly selective one productive of a fine pigment fraction sharply defined from the coarse or gritty fraction with which it was originally associated.

The aqueous suspension of fine pigment particles resulting from the centrifugal fractionating treatment hereof may be subjected to suitable treatment for the recovery of its pigment content. Thus, when it is desirable or necessary that the finished pigment be as free as possible fromwater-soluble salts or other contaminations, the aqueous suspension of fine pigment particles may advantageously undergo the continuous electrophoretic centrifugating treatment described in Lyons Patent No. 2,057,156, dated October 13, 1936. go 1 Such electrophoretic centrifugating treatment is centered about the deposition of the pigment particles from aqueous suspension on a centrifuge wall under combined electrophoretic and centrifugal forces and the progressive removal from the centrifuge wall of the solids thus deposited thereon as fresh suspension to be electrophoretically centrifuged is being progressively fed into the centrifuge. The fine pigment fraction thus recovered from suspension as a moist plastic mass may be dried, disintegrated. in a ball mill, hammermill, or equivalent instrumentality, and used as such in making lacquer, paint, and other compositions. The pigment product thus realized, being composed essentially only of particles of a size less than about 2 microns,is eminently adapted for its intended purpose by reason of the smoothness, uniformity of texture, and high opacity or coveringpower in coatings or films wherein it appears,

The relatively coarse or oversized Pigment fraction resulting herein from the centrifugating treatment of. the ball-milled slurry may be repassed into the ball mill for further bail-milling along with fresh slurry of calcined pigment fed into .the ball mill. The fraction of coarse pigment returned to the ball mill may, of course, vary, depending-upon the rate at which slurry is being run-therethrough and the other conditions determinative of the grinding efficiency thereof. Indeed, it is one of the advantages of the instant invention that the conditions of operation of the ball mill may be varied widely withoutaflecting the quality of the useful or fine pigment product proh1ciblhereby. It might be noted that the eflluent' from the electrophoretic centrifuge, which may contain some residue of pigment suspended therein, may be reused at a suitable stage in the method hereof, for instance, 0 be reused as the aqueous medium for suspending (w the titanium dioxide emerging from the calcining kiln or be recycled together with the relatively coarse or oversized pigment fraction into the ball mill In some instances it may be preferable to' apply other steps for recovering the fine'or useful pigment fraction from the aqueous suspension of the fine pigment fraction produced by the centrifugating fractionating treatment hereof.

4 '70 suitable pigment-fiocculating agent, such as magnesium sulphate, in amount controlled-to generate pigment flocks 1 a size enabling substan-c tially complete recovery 0 such flocksby or centrif I -tion, as a distinguished from electro "16 filters, in filter presses, or the like, while avoiding so far as possible excess flccculating agent whose substantially complete removal fromithe recovered pigment solids may present some difliculty. Apropos of treating the aqueous suspension of flne or useful pigment with a pigment-flocculating agent, it might be observed that, by reason of initially dispersing the pigment in water in the substantial absence of pigment-dispersing agent, such as sodium silicate, a minimum amount of; flocculating agent is necessary for developing in the suspension readily recoverable pigment flocks; and this means not only a saving in cost of fiocculating agent but also greater freedom from residual water-soluble vflocculating agent in the ,flocculated pigmentproduct recovered froznsus: pension by centrifugation or filtration. The fact is that the'product thus recovered in the substantially complete absenceof any rinsing with water, contains but a trace of water-soluble contamination, insomuch that it is useful for pigmenting various artificial resins, such as the phenolformaldehyde resins and the urea resins, without causing the discoloration therein frequently 'imputable to water-soluble ingredients in the pigment. Again, the mere trace of water-soluble pigment-flocculating agent left in the pigment products hereof is insuflicient to induce settling tendency in lacquers, paints, or like compositions into which they enter, as contrasted with the marked settling tendency in paints, lacquers, or

- the like in which there occurs pigment whose conpension into a fine fraction and a relatively tent of residual pigment-flocculating agent is relatively high.

The centrifugating fractionating treatment performed herein for the purpose of resolving the slurry of ball-milled pigment into a coarse pigment fraction returned to the ball mill and a relatively fine pigment fraction put to use aii'ords important advantages over the gravitational hydro-separatory methods heretofore frequently relied upon for separating pigments into fine arid coarse pigment fractions. In the gravitational hydro-separatory methods of the prior art, which depend essentially upon natural gravitational subsidence of the coarse pigmentparticles, while the fine pigment particles remain in suspension, it is virtually impossible to realize useful fractionation unless the pigment suspension is of relatively low solids content, say, about 8 to 12%; and

even at such low solids content, it is usually the case that. a dispersing agent, such as sodium silicate, is added to the pigment suspension to deflocculate the pigment particles and thereby to reduce the viscosity of the suspension markedly and to increase the rate of subsidence of the coarse pigment particles. In the process hereof,

, on the other hand, it is possible to perform the centrifugal fractionation of the calcined and ball-- coarse fraction,- even in the absence of any dispersing agent whatever in the suspension: *Aside from the fact that P gment suspension can be handled most economically at high solids content, by reason of the smaller amount of water passing through. the processing equipment, the

absence of sodium silicate or like'pigment-dispersing agent in the suspension ensures a finished 7,6.

pigment'productwhose particles do not tendto coalesce into aggregates, as is true of the pigment products which are derived from pigment suspensions containing sodium silicate or like pigment dispersing agent and which may hence contain residual silica gel or like cementitious material tending to bond together the pigment particles into undesirable aggregates.

The method hereof presents the important advantage that,

irrespective of variations in the pigment-calcining and ball-milling operations and variations in the ball-milled slurry, it may be controlled to give predetermined or reproducible results, i. e., pigment fractions of a particular particle size or particle size range and hence particularly adapted for certain purposes.

It-should be understood by those skilled in the art that the principles of the instant invention are applicable to the processing of other pigments or extenders, such as lithopone, zinc sulphide, calcium sulphate, barium sulphate, etc., which, like titanium dioxide, are prepared by processes involving precipitation, calcination, and ballmilling to acquire those qualities desired or required when they are used for pigmenting and/or extending paints, lacquers, resins, and congeneric compositions. Accordingly, it is intended that the various pigments or extenders that are put j through treatments similar to those employedjn the preparation of titanium dioxide shall be considered as the equivalents of titanium dioxide and that the appended claims are to be construed as being imbued with such equivalency even though titanium dioxide may be the pigment set forth specifically therein.

I claim:

1. A method of producing from precipitated titanic hydroxide, part of whose solids content consists of particles of a size smaller than the wave length of visible light, an improved titanium dioxide pigment capable of smoothly covering surfaces with uniform texture and high opacity, which comprises calcining said precipitated titanic hydroxide and thereby transforming its content of particles smaller than the wave length of light just beyond the visible range into titanium dioxide aggregates of distinctly larger size, ball-milling the resulting calcined titanium dioxide pigment in the presence of water to form a pigment suspension, continuously centrifuging the resulting pigment suspension under substantially constant solids-depositing conditions, including substantially constant efiective centrifuging force, to remove coarse pigment particles while leaving the fine pigment particles in suspension, and recovering the fine pigment particles from suspension as an improved titanium dioxide pigment of substantially constant particle size characteristics throughout the centrifuging op-- eration.

2. A method of producing from precipitated titanic hydroxide, part of whose solids content consists of particles of a size smaller than the wave length of visible light, an improved titani-- um dioxide pigment capable of smoothly covering the surfaces .with uniform texture and high opacity, which comprises calcining said precipitated titanic hydroxide and thereby transforming its content of particles smaller than the wave cluding substantially constant eflectivecentrifuging force, to remove coarse pigment particles while leaving the fine pigment particles in suspension, adding to the suspension of fine pigment particles a fiocculating agent in amount suflicient to flocculate such particles, and recovering the fiocculated pigment particles from suspension as as improved titanium dioxide pigment of substantially constant particle size characteristics throughout the centrifuging operation.

3. A method of producing from precipitated titanic hydroxide, part of whose solids content the resulting pigment suspension under substantially constant solids-depositing conditions, including constant effective centrifuging force, to

tially constant solids-depositing conditions, including substantially constant eflective centrifu ing force, to remove pigment particles coarser than about 2 microns while leaving the finer pigment particles in suspension, adding to the suspension of finer pigment particles at flocculating agent in amount sufiicient to flocculate such particles, and recovering the fiocculated pigment particles from suspension as an improved titanium dioxide pigment of substanttally constant particle size characteristics throughout the centrifuging operation. Y 6. A method of producing from precipitated tiremove coarse pigment particles while leaving the fine-pigment particles in suspension, and recovering the fine pigment particles from suspension under combined centrifugal and electrophoretic' forces as an improved titanium dioxide pigment of substantially constant particle size characteristics throughout the centrifugingoperation.

4. A method of producing from precipitated titanic hydroxide, part of whose solids content consists of particles of a size smaller than the wave length of visible light, an improved titanium dioxide pigment capable of smoothly covering surfaces with uniform texture and high opacity, which comprises calcining said precipitated titanic hydroxide and thereby transforming its content of particles smaller than the wave length of light just beyond the visible range into titanium dioxide aggregates of distinctly larger size,

ball-milling the resulting calcined titanium dioxide pigment in the presence of water to form a pigment suspension substantially devoid of pigmeat-dispersing agent, continuously centrifuging the resulting pigment suspension under substantiall'y constant solids-depositing conditions, including constant efiective centrifuging force, to remove pigment particles coarser than about 2 microns while leavingthe finer pigment particles in suspension, and recovering the finer pigment particles from suspension as an imprwed titanium dioxide pigment of substantially constant particle size characteristics throughout the centrifuging operation.

5. A method of producing from precipitated titanic hydroxide, part of whose solids content ball-milling the resulting cal tanic hydroxide, part of whose solids content consists of particles of a size smaller than the wave length of visible light, an improved titanium dioxide pigment capable of smoothly covering surfaces with uniform texture and high opacity, which comprises calcining said precipitated titanic hydroxide and thereby transforming its content of particles smaller than the wave length of light just beyond the visible range into titani-- um dioxide aggregates of distinctly larger size, ball-milling the resulting calcined titanium dioxide pigment in the presence of water to form a thick pigment suspension, continuously centrifuging the resulting pigment suspension under substantially constant solids-depositing conditions, including substantially constant effective centrifuging force, and at a solids content mark-' edly greater than about 12% to remove coarse pigment particles while leaving the fine pigment particles in suspension, and recovering the fine pigment particles from suspension as an improved titanium dioxidepigment of substantially constant particle size characteristics throughout the centrifuging operation.

7. A method of producing from precipitated titanic hydroxide, part of whose solids content consists of particles of a size smaller than the wave length of visible light, an improved titanium dioxide pigment capable of smoothly covering surfaces with uniform texture and high opacity, which comprises calcining said precipitated titanic hydroxide and thereby transforming its content of particles smaller than the wave length of light just beyond the visible range into titanium dioxide aggregates of distinctly larger size, ballmilling the resulting calcined titanium dioxide pigment in the presence of water-to form a thick pigment suspension, continuously centrifuging the resulting pigment suspension under substantially constant solids-depositing conditions, including substantially constant effective centrifuging force, and at a solids content markedly greater than about -12% to remove pigment particles coarser than about 2 microns while leaving the finer pigment particles in suspension, and recovering the finer pigment particles from suspension as an improved titanium dioxide pigment of substantially constant particle size characteristics the resulting pigment suspension under substan- L throughout the centrifuging operation.

8. A method of producing from precipitated titanic hydroxide, part of whose solids content consists-of particles of a size smaller than the wave length of visible light, an improved titanium dioxide pigment capable of smoothly covering dioxide aggregates of distinctly larger size, ballmilling the resulting calcined titanium dioxide pigment in the presence of water to form a thick pigment suspension, continuously centrifuging the resulting pigment suspension under substantially constant solids-depositing conditions, including substantially constant effective centrifuging force, and at a solids content markedly greater than about 12% to remove pigment particles coarser than about 2 microns while leaving the finer pigment particles in suspension, adding to the suspension of finer pigment particles a fiocculating agent-in amount sufficient to flocculate such particles, and recovering the fiocculated pigment particles from suspension as an improved titanium dioxide pigment of substantially constant particle size characteristics throughout the centrifuging operation. i

9. A method of producing from precipitated titanic hydroxide, part of whose solids content consists of particles of a'size smaller than the v wave length of visible light, an improved titanium dioxide pigment capable of smoothly covering surfaces with uniform-texture and high opacity, which comprises calcining said precipitated titanic hydroxide and thereby transforming its content of particles smaller than the wave length of light just beyond the visible range into titanium dioxide aggregates of distinctly-larger size, ball:

.milling the resulting calcined "titanium dioxide which comprises calcining said precipitated tie I tanic hydroxide and thereby transforming its;

pigment in the presence of water to form a thick pigment suspension, continuously -.centrifuging the resulting pigmentsuspensionunder substantially constant solids-depositing conditions, in-

cluding substantially constant effective centri fuging force, and at a solids content markedly greater than about-12% to remove pigment particles coarser than about 2 microns while leaving the finer pigment particles in suspension, and recovering the finer pigment particles from suspension under combined centrifugal and electrophoretic forces as an improved titanium, dioxide pigment of substantially-constant 'particle size characteristics throughout the centrifuging fo eration. i

10. A method of producing from precipitated,

titanic 'hydroxide, part ,of whose solids content consists of particles of a size smallerlthan the wave length of visible light, an improved titanium dioxide pigment capable of smoothly covering sur-" faces with uniform texture and high opacity,

content of particles smaller than the wave length of light just beyond the visible range into titanium dioxide aggregates of distinctly larger size, ballmilling the resulting calcined titanium dioxidepigment in the presence of water to form a thick pigment suspension substantially devoid of pigment-dispersing agent, continuously centrifuging the resulting pigment suspension under substantially constant solids-depositing conditions, ineluding substantially constant eflective centrifuging force, and at a solids content-markedly greater than about 12% to remove pigment particles coarser than about 2 microns while leav-"v ing the finer pigment particles in suspension, fiocculating the finer pigment particles left in suspension, dewatering the fiocculated pigment particles to substantial dryness, and disintegratv ing the dried pigment mass to provide an improved titanium dioxide pigment of substantiallyconstant size characteristics throughout the centrifuging operation. Q

11. A method of producing from precipitated titanic hydroxide, part of whose solids content consists of particles of a size smaller than the wave length of visible light, an improved titanium dioxide pigment capable of smoothly covering surfaces with uniform texture and high opacity, which comprises calcining said precipitated titanic hydroxide and thereby transforming its content of particles smaller than the wave length of light just beyond the visible range into titanium dioxide aggregates of distinctly largersize, ballmilling the resulting calcined titanium dioxide pigment in the presence of water to form a thick pigment suspension substantially devoid of' pigment-dispersing agent, continuously centrifuging the resulting pigment suspension under substantially constant solids-depositing conditions, including substantially constant effective centriiuging force, and at a solids content markedly greater than about 12% to remove pigment particles coarser than about 2 microns while leaving the finer pigment-particles in suspension,

fiocculating the finer pigment particles left in suspension, filtering thefiocculated pigment particles, drying the filtered pigmentmass, and disintegrating the dried mass to provide an improved titanium dioxide pigment of substantially constant particlesize characteristics throughout the centrifuging operation.

12. A method which comprises continuously feeding calcined titanium oxide pigment and 'water through a ball mill while ball-milling such pigment to produce an aqueous slurry containing a pigment fraction of oversized particles and a pigment fraction of the desired fine particles,

continuously centrifuging the aqueous slurry of ball-milled pigment under substantially constant solids-depositing conditions. including substantially constant effective centrifuging force,"to remove continuously therefromthe pigment fraction of oversized particles and to produce con-. tinuously a.suspension of an improved titanium dioxide pigment of substantially constant par- ,ticle'size characteristics throughout the centrifuging operation and continuously feedingv substantially only such removed pigment fraction together with newly calcined titanium oxide pigment. and water being fed through the'ball mill. 13. A method which comprises continuously feedingcalcined titanium oxide ,pigment and water? "through'la ball mill whileball-milling suchpigment to produce an aqueous slurry cont i' i hg a pigment fraction of oversized parti- ,cles and a ,pignient fraction of the desired fine particles, continuously centrifuging the aqueous slurry of ball-milled pigment under substantially constant solidsdepositing conditions, including substantially constant effective centrifuging force, and at a solids content of about} 30% to 45% to remove continuously therefrom the pigment fraction of oversized particles .and to produce continuously a suspension of an improved titanium dioxide pigment of substantially constant paring conditions, including substantially constant. efiective centrifuging force, and at a solids content upwards of about 30% to remove continuous- 1y therefrom the pigment fraction of oversized particles and to produce continuously a suspension of an improvedtitanium dioxide pigment of substantially'consta'nt particle size characteristics throughout the centrifuging operation and continuously feeding substantially only such removed pigment fraction together with newly calcined titanium oxide pigment and water beingfed through the ball mill.

15.- A method which comprises continuously 'feeding calcined titanium oxide pigment and water through a ball mill while ball-milling such pigment to produce an aqueous slurry containing upwards of about 30% solids as both a pigment fraction of oversized particles and a pigment fraction of the desired fine particles, continuously centrifuging the aqueous slurry of ball-milled pigment under substantially constant solids-depositing conditions, including substantially constant effective centrifuging force, and at a solids con- 

